Binoculars

Binoculars, often called field glasses, are optical instruments consisting of two refracting telescopes mounted side-by-side. They are aligned to point in the same direction so that a user can employ binocular vision. This means the viewer uses both eyes simultaneously to look at distant objects. Unlike a monocular telescope, binoculars provide a three-dimensional image. This happens because each eyepiece presents a slightly different image to each eye. The brain's visual cortex then uses this parallax to generate a sense of depth.

To understand how they function, one must look at the three main optical assemblies. The first is the objective lens assembly located at the front of the device. This assembly gathers light from an object and forms an image at the image plane. The second part is the image orientation correction assembly, which is usually a prism system. This assembly is vital because it shortens the optical path and prevents the image from appearing inverted or laterally reversed. The third part is the eyepiece lens assembly located near the eyes. This assembly's primary function is to magnify the image for the viewer.

Optical designs have changed significantly since the 17th century. Early binoculars utilized Galilean optics, which consist of a convex objective lens and a concave eyepiece lens. The Galilean design keeps the image upright, but it suffers from a narrow field of view and low magnification. Because they are short and lightweight, these are still used today for opera glasses or jeweler's loupes. Later, Keplerian optics were developed to provide higher magnification. This design uses a positive eyepiece lens, but it produces an inverted image. To fix this, early makers used relay lenses to erect the image, though this made the binoculars very long.

Prism technology eventually revolutionized the industry by allowing for shorter instruments. Ignazio Porro patented an image-erecting system in 1854 that used a Z-shaped configuration. These Porro prism binoculars have widely separated objective lenses, which provides a superior sensation of depth. They also fold the optical path so the physical length is shorter than the objective's focal length. In 1894, the Carl Zeiss company introduced improved modern Porro prism binoculars. This was possible because Ernst Abbe and Otto Schott developed high-quality Crown glass.

Another major design is the roof prism binocular, which features lenses that are almost in a straight line. This makes the instrument much narrower, more compact, and lighter than Porro prism models. Most roof prisms use either the Schmidt–Pechan design or the Abbe–Koenig design. The Schmidt–Pechan design was invented in 1899, while the Abbe–Koenig design was patented by Carl Zeiss in 1905. While roof prisms are popular, they are harder to manufacture. The prism angles must be correct within 2 arcseconds to avoid a double image.

Manufacturing precision is a critical factor in binocular quality. Porro prism binoculars require alignment tolerances within 10 arcminutes during factory collimation. High-quality Porro designs may even include grooves on the prism hypotenuse to reduce reflections. Roof prism binoculars require even tighter tolerances and are often permanently fixed to a metal plate to maintain alignment. Historically, Porro prism binoculars offered better resolution and contrast. However, since the invention of phase correction coatings in 1988, high-quality roof prism binoculars can achieve similar performance.

When reading binocular specifications, two main parameters are listed on the cover plate. The first is magnification, which is the ratio of the objective focal length to the eyepiece focal length. For example, 7x magnification means the image appears seven times larger. Most handheld binoculars range from 7x to 10x to minimize the effects of hand tremors. The second number is the objective diameter, which measures the front lens size. A larger diameter, such as 50 mm compared to 25 mm, allows the device to gather more light. This results in a brighter and sharper image for the user.